This invention concerns a rolling device such as a rolling bearing, a ball screw and a linear guide and, more in particular, it relates to a rolling device suitable to use, for example, in semiconductor production apparatus, chemical fiber production machines, liquid crystal panel production apparatus, and equipments using electron beams or X-rays.
In rolling bearings such as ball bearings, bearing rings and rolling elements thereof are generally formed of iron and steel materials such as high carbon chromium bearing steels and case hardening steels and rolling bearings are used in various working circumstances. Accordingly, in machines that use water or sea water such as food machines or chemical fiber production machines, since rust is formed on the surfaces of bearing rings and rolling elements due to water or sea water intruding into the inside of bearings making them no more usable, rolling bearings in which bearing rings or rolling elements are formed of austenitic stainless steels such as SUS 440C are mainly used.
While such rolling bearings have good corrosion resistance to water or sea water, corrosion resistance to chemicals, for example, acidic solutions such as sulfuric acid or alkaline solutions can not be said favorable. In view of the above, rolling bearings in which bearing rings are formed of titanium alloys have often been used in machines that use chemicals, for example, acidic solutions such as sulfuric acid or alkaline solutions, for example, semiconductor production apparatus and liquid crystal panel production apparatus. However, titanium alloys lack in the surface hardness when merely applied with usual solution treatment or aging treatment and can not be used as they are as the material for the bearing ring of the rolling bearing. Accordingly, when titanium alloy is used as the material for the bearing ring of the rolling bearing, it is necessary to increase the surface hardness of the titanium alloy by some or other methods.
As a method of increasing the surface hardness of the titanium alloys, a method of increasing the surface hardness of xcex2-titanium alloy to the surface hardness of Hv 600 or more is disclosed in Japanese Published Unexamined Patent Application No. Hei 11-22221. However, according to the method disclosed in this publication, xcex1-phase has to be precipitated in excess in the xcex2 type titanium alloy and since the xcex1-phase is poor in the corrosion resistance compared with the xcex2-phase, the corrosion resistance is sometimes insufficient depending on the working circumstance. Further, in the method disclosed in this publication, it is necessary to apply shot peening after cold working to result in a problem of increasing the cost by the increase in the number of steps such as cold working or shot peening.
Further, in equipments used in semiconductor production steps, for example, an electron beam lithography system or a wafer inspection apparatus, laser beams have been used so far as a means for writing circuit patterns on a wafer but along with the micro-miniaturization of the circuit patterns, electron beams having shorter wavelength and higher resolution power than the laser beams have been used in recent years. In the electron beam lithography system or the wafer inspection equipment using the electron beams, the electron beams are deflected easily when disturbance is present in peripheral magnetic fields to sometimes lower the writing accuracy or inspection accuracy. Accordingly, in a case of using rolling bearings to such apparatus, it requires such a rolling bearing that does not disturb the peripheral magnetic fields by the rotation of the bearing ring and, in order to satisfy such a demand, use of non-magnetic stainless steel or beryllium copper as the material for the bearing ring of the rolling bearing has been investigated.
However, since the permeability of the non-magnetic stainless steel is about 1.04 to 1.002, when the non-magnetic stainless steel is used as the material for the bearing ring of the rolling bearing, it has a possibility of causing disturbance in the peripheral magnetic fields when the bearing ring is magnetized even slightly. On the other hand, beryllium copper has a permeability of 1.001 or less and has less possibility of causing disturbance in the peripheral magnetic fields as in the non-magnetic stainless steel. However, since a portion of elements or compounds thereof constituting beryllium copper is considered as environmental load substances, its use may sometimes suffer from restriction. Further, since it is expected that the environmental problem will be considered more important in the future, use of beryllium copper itself may possibly be limited. Further, since beryllium copper has a highest hardness of about Hv 400, it involves a problem of tending to cause early wear during use under large load.
A first object of the present invention is to provide a rolling device that can be used favorably over an extended period of time even in a highly corrosive circumstance.
A second object of the present invention is to provide a rolling device suitable to use in machines that use strongly acidic solutions such as sulfuric acid or strongly alkaline solutions.
A third object of the present invention is to provide a rolling device that can be used favorably over an extended period of time also in a circumstance where non-magnetic property is required.
A fourth object of the present invention is to provide a rolling device suitable to use in equipments using electron beams or X-rays such as a wafer inspection apparatus or nuclear magnetic resonance diagnostic apparatus.
A fifth object of the present invention is to provide a rolling device that can be used favorably over an extended period of time also in a circumstance where a lubricant such as grease can not be used.
A rolling device according to this invention comprises an outer member and an inner member each having a raceway surface and rolling elements rolling on the raceway surface by rotational or linear movement of the outer member or the inner member in which the outer member and/or the inner member is constituted with at least one kind of titanium alloys of xcex2 type titanium alloys, near xcex2 type titanium alloys and xcex1+xcex2 type titanium alloys.
In a preferred embodiment of the present invention, the titanium alloy has a surface hardness of Hv 400 or more and less than Hv 600. Further, the titanium alloy has a core hardness of Hv 420 or more, preferably, Hv 450 or more and has an oxygen compound layer on the surface in which the oxygen compound layer comprises a titanium oxide containing rutile type TiO2 and has a thickness of 20 nm or more and, preferably, 50 nm or more.
In a preferred embodiment of the present invention, the rolling element is constituted with at least one kind of materials of titanium alloys, silicon nitride, silicon carbide, zirconia series ceramics, alumina series ceramics and SIALON series ceramics.
In another preferred embodiment of the present invention, the rolling device further comprises a cage for holding the rolling elements and the cage has a heat conductivity of 20 W/(mxc2x7K) or more. Further, the cage is constituted, preferably, with one kind of materials of copper, tellurium copper, brass, aluminum bronze, phosphorus bronze, nickel silver, cupro nickel and beryllium copper.
In a further preferred embodiment of the present invention, at least one of the outer member, the inner member and the rolling elements is constituted with a titanium alloy and the titanium alloy has a xcfx89 phase with the size of the crystal particles of 1 xcexcm or less, preferably, 800 nm or less and, further preferably, 10 nm or less.
In a further preferred embodiment of the present invention, the outer member and/or the inner member has a hard film on the raceway surface. The hard film is constituted with at least one kind of materials of TiN, TiC, TiCN, TiAlN, CrN, SiC and diamond-like carbon, and the raceway surface formed with the hard film has a surface hardness of Hv 350 or more and, preferably, Hv 450 or more. Further, the outer member and/or the inner member has a lubricating film of 0.1 xcexcm to 10 xcexcm and, preferably, 0.1 xcexcm to 5 xcexcm on the hard film.
In a further preferred embodiment of the present invention, the rolling element is constituted with a superhard alloy and or cermet and has a heat conductivity of 35 W/(mxc2x7K) or more, preferably, 50 W/(mxc2x7K) or more.
In a further preferred embodiment of the present invention, the rolling element is constituted with an iron and steel material and has the surface hardening layer having corrosion resistance on the surface and the surface hardening layer is formed by applying a chromium diffusion penetration treatment or a nitridation treatment to the surface of a base material constituting the rolling element.
In a further preferred embodiment of the present invention, the titanium alloy is a titanium alloy satisfying the condition: 3.7xe2x89xa6(H/E), preferably, 4.0xe2x89xa6(H/E) and, further preferably, (H/E)xe2x89xa64.5 where E (Gpa) represents the Young""s modulus and H (Hv) represents the minimum hardness for the portion from the raceway surface to a depth corresponding to {fraction (2/100)} to {fraction (5/100)} for the diameter of the rolling element.
In a further preferred embodiment of the present invention, the ratio xcex12/xcex11 between the heat expansion coefficient xcex11 of the titanium alloy and the heat expansion coefficient xcex12 of the rolling element is within a range of 0.4 to 1.3.
In a further preferred embodiment of the present invention, a sealed plate for shielding an opening formed between the outer member and the inner member is formed of titanium at a purity of 99.5% or higher and the outer member and the inner member each has an oxide film comprising TiOx (in which 0 less than x less than 2) on the surface.
The xcex2-type titanium alloy and the xcex1+xcex2 type titanium alloy increase the hardness by fine precipitation of the xcex1-phase in the xcex2-phase by applying a solution treatment to the titanium alloy from the vicinity of the temperature at which the xcex1-phase transforms into the xcex2-phase to transform the metal structure substantially into the xcex2-phase and then applying an aging treatment to the titanium alloy. However, when the xcex1-phase is precipitated by the aging treatment, xcex2-stable alloying elements are concentrated in the xcex2-phase along with preparation of the xcex1-phase. Accordingly, local corrosion tends to occur due to the difference of the corrosion resistance between the xcex1-phase and the xcex2-phase along with increase in the precipitation amount of the xcex1-phase. Accordingly, it is necessary that the xcex2-phase in the xcex2-type titanium alloy or the xcex1+xcex2 type titanium alloy remains to some extent in order that the alloy can be used suitably also in a highly corrosive circumstance but, since the xcex2-phase is soft compared with the xcex1-phase, the wear resistance is insufficient when the amount of the xcex2-phase is excessive while the corrosion resistance is improved.
The present inventors have made an earnest study on the solution treatment and the aging treatment of the titanium alloy and have found that a titanium alloy which is satisfactory as the material for the bearing ring of a rolling element can be obtained by applying a low temperature oxidation treatment to the titanium alloy after the solution treatment such that the surface hardness of the titanium alloy is Hv 400 or more and less than Hv 600. Then, since the hardness of Hv 400 or more and less than 600 is a hardness comparable with that of the stainless steels such as SUS 630 or YHD50 (trade mark) used so far as the bearing material for the special circumstances, it can be used sufficiently as the material for the bearing element in a circumstance where a large load is not applied by so much.
The surface of a portion of the bearing ring that is in contact with the rolling element has an elliptic shape that is referred to as a contact ellipse and the area is extremely small. Accordingly, when stress is applied to the bearing ring, an extremely large surface pressure exerts on the contact ellipse. When the bearing ring of the rolling bearing is formed of a titanium alloy (Young""s modulus: about 100 Gpa) and the rolling element is formed of a stainless steel (Young""s modulus: about 200 Gpa), this means that the bearing ring deforms more greatly than the rolling element and the area of the contact ellipse in contact with the rolling element increase. In view of calculation, the area of the contact ellipse of a bearing ring made of titanium alloy is larger than that of the bearing ring made of a stainless steel and the maximum contact surface pressure at the contact ellipse of the bearing ring made of titanium alloy is about 0.8 times that of the bearing ring made of stainless steel. Accordingly, since the contact area with the rolling element is larger in the bearing ring made of the titanium alloy than in the bearing ring made of the stainless steel, the contact pressure surface is lowered and the rolling fatigue is moderated preferably.
However, when the surface hardness of the bearing ring made of the titanium alloy is less than Hv 400, wear tends to be caused abruptly even when the surface pressure is low. Further, indentations are tend to be caused upon intrusion of obstacles such as dusts to shorten the life of the rolling bearing. Accordingly, it is necessary for the bearing ring made of the titanium alloy that the surface hardness is Hv 400 or more and the surface hardness of the bearing ring made of the titanium alloy is more preferably Hv 450 or more when higher wear resistance is required. Further, when corrosion resistance or wear resistance is further required, the titanium alloy can be provided with higher hardness and corrosion resistance by surface hardening heat treatment such as a nitridation treatment or an oxidation treatment.
The permeability of a titanium alloy is 1.001 or less and the value is nearly equal with that for the substantially complete non-magnetic property. Accordingly, since peripheral magnetic fields suffer from no effects by the rotation of the bearing ring, it can be used favorably to equipments using electron beams or X-rays. However, if the rolling element or the cage is not a non-magnetic body, magnetization thereof causes deterioration of the accuracy in the apparatus described above by magnetization thereof. Accordingly, when a substantially complete non-magnetic property is required for the rolling device, it is necessary that the permeability of the rolling element and the cage should also be 1.001 or less like that in the permeability of the bearing ring made of the titanium alloy.
The material for the rolling element with the magnetic permeability of 1.001 or less can include titanium alloys, as well as ceramics such as silicon nitride, silicon carbide, zirconia series ceramics, alumina series ceramics and SIALON series ceramics or titanium alloys. Further, the material for the cage with the permeability of 1.001 or less can include resins such as polyamide and fluoro resins or non-magnetic metals such as brass and SUS 304.
When the materials are investigated in details, in the cage made of stainless steels typically represented by SUS 304, martensite is formed by strain induced transformation upon pressing. Accordingly, the cage is tended to be magnetized to result in a possibility of increasing the magnetic field fluctuation due to the rotation of the cage. Further, in recent years, specific permeability lower than 1.01 to 1.1 of the non-magnetic stainless steel, specifically, about 1.001 is demanded and the use of the cage made of the non-magnetic stainless steel is sometimes restricted. Accordingly, it is desirable that the rolling element is made of ceramics, while the cage is made of a copper series alloy.
The titanium alloy is a substantially complete non-magnetic body and ceramics are also complete non-magnetic body. On the other hand, the copper alloy is a non-magnetic material with the permeability lower than that of the non-magnetic stainless steel and the specific permeability thereof is 1.001 or less. Accordingly, even when it is used under a magnetic circumstance, since rotation of the cage does not cause fluctuation of magnetic fields, it is suitable as a cage made of metal in a rolling device used under a non-magnetic circumstance.
Further, since the copper alloy has self-lubricity, friction characteristics at the contact surface with the rolling element and at the guiding surface of the bearing ring are improved and the amount of wear is small even under a circumstance where a lubricating oil or grease can not be used or a non-magnetic and vacuum circumstance as in electron beam equipments or semiconductor production apparatus.
Further, since the cage made of the copper alloy has a high heat conductivity and causes no heat accumulation on the sliding guide surface, adhesive wear can be suppressed. Further, the copper alloy has high heat dissipation, can promote heat dissipation along with rotation of the cage and can suppress the temperature elevation of the bearing. On the contrary, in a case where the bearing is made of the titanium alloy and the cage is made of austenitic non-magnetic stainless steel such as SUS 304, since the heat conductivity and the specific heat of SUS 304 are small, temperature locally rises remarkably at a sliding portion between the cage and the bearing ring guide surface tending to cause adhesive wear relative to the bearing ring. Since the heat conductivity of the austenitic non-magnetic stainless steel such as SUS 304 is 16 W/(mxc2x7K), it is preferred, for the cage made of the copper alloy to use a cage made of a copper alloy having a heat conductivity of 20 W/(mxc2x7K) or more and, more preferably, 35 W/(mxc2x7K) or more.
Referring to the kind of the copper alloy, any of copper alloys can be used suitably so long as it has the heat conductivity of 20 W/(mxc2x7K) or more, for example, copper alloy castings such as pure copper, tellurium copper, brass, freely cutting brass, high strength brass, and aluminum bronze or stretchable copper alloys such as pure copper, tellurium, phosphorus bronze, nickel silver and cupro nickel or precipitation hardening type beryllium copper. However, since low alloys such as pure copper and tellurium copper have low strength and hardness, it is desirable to use copper alloys excluding them in a case where particular importance is attached to the wear resistance.
It is considered that the surface treatment such as an oxidation treatment or a nitridation treatment should be applied at a high temperature of 600xc2x0 C. or higher for insuring the thickness of the compound layer and diffusion promotion of intruded elements. However, when the titanium alloy is heated in oxygen or oxygen-containing gas for a predetermined period of time, since titanium has a high affinity with oxygen, an oxygen compound such as TiO2 or Ti3O is formed on the surface even at a relatively low temperature of 400 to 600xc2x0 C.
The oxygen compound such as TiO2 formed on the surface of the titanium alloy by the oxidation treatment is a highly chemically stable substance. On the other hand, the surface of the titanium compound tends to become highly reactive by sliding movement with the rolling element or the like, by which adhesive wear tends to be caused so that it is considered to be poor in the wear resistance. However, since the surface is covered with the highly chemically stable compound by applying the oxidation treatment to the titanium alloy, surface activation is suppressed and, as a result, seizure less occurs to improve the sliding property and the wear resistance.
Further, when the thickness of the oxygen compound layer formed on the surface of the titanium alloy by the oxidation treatment is 20 nm or more, the load carrying capacity increases to remarkably improve the effect of the wear resistance and sliding property. However, when the thickness of the oxygen compound layer is less than 20 nm, the effect of improving the wear resistance and the sliding property is small. Accordingly it is desirable that the thickness of the oxygen compound layer is 20 nm or more. Further, for obtaining better wear resistance and sliding property, it is preferred that the thickness of the oxygen compound layer is 50 nm or more.
When the titanium alloy is put to the oxidation treatment at a high temperature of 700xc2x0 C. or higher, the oxygen compound layer formed on the surface of the titanium alloy mainly comprises rutile type TiO2 and the thickness of the oxygen compound layer also increases. Accordingly, durability to great load is improved, but the surface roughness of the titanium alloy is sometimes deteriorated on the other hand to increase the rotational torque of the bearing.
On the other hand, when the titanium alloy is put to the oxidation treatment at a temperature of 400 to 600xc2x0 C., the oxygen compound layer formed on the surface of the titanium alloy is in a state where TiOx oxide such as rutile type TiO2 and Ti3O (x:0 less than x less than 2) and Ti are present together, which is more dense compared with the oxygen compound layer mainly comprising rutile type TiO2. Accordingly, the surface roughness after the oxidation treatment is favorable and, as a result, the rotational torque of the bearing is lowered and detachment of the compound layer or the like is less caused.
FIG. 10A shows a heat treatment step for the solution treatment and the aging treatment conducted generally as a method of hardening the xcex2 type titanium alloy and the xcex1+xcex2 type titanium alloy. In this heat treatment method, since the titanium alloy tends to be oxidized abruptly, heating is often conducted in a high vacuum atmosphere or in an inert gas atmosphere such as argon.
FIG. 10B shows a gas oxidation treatment at high temperature. In this case, it is often used after the oxidation treatment as it is but, since heating is conducted at a high temperature for a long time without the solution treatment and the aging treatment, the core hardness is lowered to sometimes give undesired effects on the rolling life. Further, as described above, it may be a worry of causing degradation in the surface roughness and brittlement of the compound layer.
FIG. 10C shows a method of an oxidation treatment at a low temperature of 400 to 600xc2x0 C. for the titanium compound after the solution treatment. Since the temperature for the oxidation treatment of 400 to 600xc2x0 C. is within a range of the temperature identical to that upon aging treatment after the solution treatment of the xcex2 type titanium alloy and the xcex1+xcex2 type titanium alloy, it can serve both as the oxidation treatment and the aging treatment. Accordingly, it does not increase the cost due to the increase in the number of steps.
Further, since the hardness is improved to Hv 420 or more by the aging treatment not only for the surface of the titanium alloy but also for the core of the titanium alloy, the rolling life is improved. Further, since the processing temperature is low, thermal deformation is small to give less possibility of deteriorating the dimensional accuracy of the bearing ring. However, when the core hardness is less than Hv 420, since the effect for extending the rolling life of the bearing is small even when it has the oxygen compound layer on the surface, it is desirable that the hardness of the core obtained by the oxidation treatment also serving as the aging treatment is Hv 420 or more. Further, for obtaining a more stable extending effect of the rolling life, it is preferred to increase the core hardness of the titanium alloy to Hv 450 or more.
The oxidation treatment is conducted in a gas atmosphere such as in oxygen or an oxygen-containing gas. For example, it is conducted in atmospheric air, in a 90% N2+10% O2 gas or in a gas in which a predetermined amount of H2O gas is mixed to an Ar gas. However, the kind of the gas in the oxidation treatment atmosphere is not restricted so long as the oxygen compound layer containing rutile type TiO2 and having a thickness of 20 nm or more can be formed on the surface by applying the oxidation treatment. Further, in order to prevent abrupt oxidation, the oxidation can also be conducted using the gas described above in a state of reducing the pressure in the heating furnace.
As the titanium alloy for which the oxidation treatment is applied, xcex1+xcex2 titanium alloy or xcex2 type titanium alloy that increases the hardness by the solution treatment and the aging treatment can be used suitably. They include, for example, Ti-6Al-4V, Ti-15V-3Cr-3Sn-3Al, Ti-22V-4Al and Ti-15Mo-5Zr-3Al. There is no restriction on the kind so long as the titanium alloy forms an oxygen compound layer containing rutile type TiO2 and having a thickness of 20 nm or more on the surface with the core hardness being Hv 420 or more by applying the oxidation treatment.